1. Field of the Invention
The invention belongs in the class of devices for measuring the thickness of films on bases as a function of the apparent magnetic attraction, i.e. adhesive force, between the surface of the film to be measured and a permanent magnet.
2. Discussion of the Related Art
German Patent DE 26 38 48 C2 discloses a magnetic film thickness measuring device in which the adhesive force on the film to be measured exerted by a permanent magnet, rotatably arranged on a measuring arm mounted at the center of gravity and hence counter-balanced, serves as a measure of film thickness. The measurement is taken by means of a spring which is tightened by rotating a drive element. The device has a dial divided into layer thicknesses, a marker being provided on the housing of the device for taking a reading, as well as a locking member between the measuring arm and dial which sets the dial depending on the movement of the permanent magnet as it pulls away.
Thickness-measuring devices of this type have a tendency to produce differences in the values of measured film thicknesses due to the fact that the dial, which increases the spring tension as it is rotated to lift the permanent magnet off the film to be measured as the adhesive force is overcome, is difficult to halt exactly at the instant when the permanent magnet is lifted. In addition, because the dial is rotated manually, the rotation speed is not always the same. This causes discrepancies in the measurement values when taking a series of measurements. The problem cannot be readily eliminated using spring bias technology.
A known approach to simplify the measuring procedure and improve the measuring accuracy of such thickness-measuring devices is to provide the rotatable drive element with a mechanical locking mechanism with gears and a high-speed flywheel of the type used in a clock mechanism as illustrated in U.S. Pat. No. 4,164,707 issued to N. Nix. In this device, an escapement like locking member is arranged between the rear lever arm of the counter-balanced rotary system and the high-speed flywheel so that the high-speed flywheel is checked as the drive element is tightened and not released until the instant at which the lever arm of the rotary system incorporating the permanent magnet is placed on the film to be measured. Once the high-speed flywheel is released, the drive element for the rotary system is displaced by the pre-stressed spring during rotation at a constant speed until the attractive force of the permanent magnet is overcome and the latter is abruptly lifted from the film being measured. At this instant, the high-speed flywheel is also halted by the locking member disposed on the rear lever arm. A reading of the measuring film thickness can then be taken from the dial.
Thickness measuring devices with an automatic locking mechanism of this type, which is activated at the instant at which the permanent magnet is lifted from the film being measured, are expensive to produce and require a lot of maintenance.
The primary objective of the invention is to provide an inexpensive thickness-measuring device with improved handling and measuring accuracy.
In a first advantageous embodiment, a magnetically soft compensating element attached to the rotary system of the permanent magnet is attracted to or repelled by the magnetic field of the electromagnet.
Alternatively, in another embodiment, a magnetically hard compensating element may be attached to the rotary system of the permanent magnet and attracted to or repelled by the magnetic field of the electromagnet.
In another advantageous embodiment, the measuring device may also be designated so that the variable magnetic field of a coil acts directly on the permanent magnet. This may be set up by arranging the electromagnet in the housing of the measuring device, coaxially with and above the permanent magnet.
However, the electromagnet may also be affixed to the rotary system and act on a magnetically soft or hard body which is stationary relative to the rotary system.
In another alternative embodiment, the electromagnet attached to the rotary system may be displaced between two permanent magnets which are stationary relative to the point of rotation.
According to the present invention, there is provided a thickness-measuring device for films in which the film thickness is determined from the adhesive force of a permanent magnet on the surface of the film to be measured. The device comprises a rotary system mounted at its center of gravity, a permanent magnet carried on the rotary system and an electromagnet. The adhesive force of the permanent magnet is overcome by the variable force of the magnetic field of the electromagnet acting on the rotary system and the value thereof as the permanent magnet is lifted off the film is used as a measure of film thickness.
A thickness measuring device embodying the present invention has an advantage over the prior art in that the force required to overcome the adhesive force of the permanent magnet on the film to be measured does not have to be applied by means of a spring and complicated mechanism for halting the element driving the rotary system at the instant the permanent magnet is lifted from the film to be measured.
In a preferred embodiment, the variable current which activates the electromagnet is used as a measurement of film thickness. Furthermore, the value of the compensation force as the permanent magnet is pulled away is advantageously converted into film thickness by means of an electronic system and can be indicated on a display as a readout depicting physical dimensions.
Particularly high measuring accuracy is obtained by detecting the point on the current curve of the electromagnet when the permanent magnet is pulled away from the film to be measured. This point is readily identified by the fluctuation in the current curve caused by the sudden movement of the electromagnet coil relative to a core at the point of release of the permanent magnet from the film being measured. The current value may be converted into film thickness, however, the fluctuations generated at separation are current level fluctuations which can cause erroneous readings. To avoid this error source, the electronic system generates a linearly increasing current application to the electromagnet and measures the time from first current application to fluctuation and that time value is what is actually interpreted as film thickness. As a result, the invention offers another feature whereby the time to the signal in the current curve, generated as the permanent magnet is pulled away from the film, is used to cut off the current to the electromagnet.